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SubjectRe: [PATCH] KVM: arm64: Treat emulated TVAL TimerValue as a signed 32-bit integer
Hi Alexandru,

On 2020-01-27 10:36, Alexandru Elisei wrote:
> According to the ARM ARM, registers CNT{P,V}_TVAL_EL0 have bits [63:32]
> RES0 [1]. When reading the register, the value is truncated to the
> least
> significant 32 bits [2], and on writes, TimerValue is treated as a
> signed
> 32-bit integer [1, 2].
>
> When the guest behaves correctly and writes 32-bit values, treating
> TVAL
> as an unsigned 64 bit register works as expected. However, things start
> to break down when the guest writes larger values, because
> (u64)0x1_ffff_ffff = 8589934591. but (s32)0x1_ffff_ffff = -1, and the
> former will cause the timer interrupt to be asserted in the future, but
> the latter will cause it to be asserted now. Let's treat TVAL as a
> signed 32-bit register on writes, to match the behaviour described in
> the architecture, and the behaviour experimentally exhibited by the
> virtual timer on a non-vhe host.
>
> [1] Arm DDI 0487E.a, section D13.8.18
> [2] Arm DDI 0487E.a, section D11.2.4
>
> Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>

Huhuh... Nice catch!

Fixes: 8fa761624871 ("KVM: arm/arm64: arch_timer: Fix CNTP_TVAL
calculation")

(how many times are we doing to fix this???)

> ---
> include/kvm/arm_arch_timer.h | 2 ++
> virt/kvm/arm/arch_timer.c | 3 ++-
> 2 files changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/include/kvm/arm_arch_timer.h
> b/include/kvm/arm_arch_timer.h
> index d120e6c323e7..be912176b7a3 100644
> --- a/include/kvm/arm_arch_timer.h
> +++ b/include/kvm/arm_arch_timer.h
> @@ -10,6 +10,8 @@
> #include <linux/clocksource.h>
> #include <linux/hrtimer.h>
>
> +#define ARCH_TIMER_TVAL_MASK ((1ULL << 32) - 1)
> +
> enum kvm_arch_timers {
> TIMER_PTIMER,
> TIMER_VTIMER,
> diff --git a/virt/kvm/arm/arch_timer.c b/virt/kvm/arm/arch_timer.c
> index f182b2380345..5d40f17f7024 100644
> --- a/virt/kvm/arm/arch_timer.c
> +++ b/virt/kvm/arm/arch_timer.c
> @@ -805,6 +805,7 @@ static u64 kvm_arm_timer_read(struct kvm_vcpu
> *vcpu,
> switch (treg) {
> case TIMER_REG_TVAL:
> val = timer->cnt_cval - kvm_phys_timer_read() + timer->cntvoff;
> + val &= ARCH_TIMER_TVAL_MASK;

nit: Do we really need this mask? I'd rather see it written as

val = lower_32_bits(val);


> break;
>
> case TIMER_REG_CTL:
> @@ -850,7 +851,7 @@ static void kvm_arm_timer_write(struct kvm_vcpu
> *vcpu,
> {
> switch (treg) {
> case TIMER_REG_TVAL:
> - timer->cnt_cval = kvm_phys_timer_read() - timer->cntvoff + val;
> + timer->cnt_cval = kvm_phys_timer_read() - timer->cntvoff + (s32)val;
> break;
>
> case TIMER_REG_CTL:

Otherwise, looks good to me. If you're OK with the above change, I'll
take it as a fix.

Thanks,

M.
--
Jazz is not dead. It just smells funny...

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